Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available, and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, generator, gearbox, nacelle, and one or more rotor blades. The rotor blades capture kinetic energy of wind using known airfoil principles. For example, rotor blades typically have the cross-sectional profile of an airfoil such that, during operation, air flows over the blade producing a pressure difference between the sides. Consequently, a lift force, which is directed from a pressure side towards a suction side, acts on the blade. The lift force generates torque on the main rotor shaft, which is geared to a generator for producing electricity.
During operation, wind impacts the rotor blades and the blades transform wind energy into a mechanical rotational torque that rotatably drives a low-speed shaft. The low-speed shaft is configured to drive the gearbox that subsequently steps up the low rotational speed of the low-speed shaft to drive a high-speed shaft at an increased rotational speed. The high-speed shaft is generally rotatably coupled to a generator so as to rotatably drive a generator rotor. As such, a rotating magnetic field may be induced by the generator rotor and a voltage may be induced within a generator stator that is magnetically coupled to the generator rotor. The associated electrical power can be transmitted to a main transformer that is typically connected to a power grid via a grid breaker. Thus, the main transformer steps up the voltage amplitude of the electrical power such that the transformed electrical power may be further transmitted to the power grid. Further, the main transformer is oftentimes a medium-voltage transformer.
In many wind turbines, the generator rotor may be electrically coupled to a bi-directional power converter that includes a rotor-side converter joined to a line-side converter via a regulated DC link. More specifically, some wind turbines, such as wind-driven doubly-fed induction generator (DFIG) systems or full power conversion systems, may include a power converter with an AC-DC-AC topology.
FIG. 1 illustrates a simplified schematic diagram of an electrical system 10 of a conventional wind turbine illustrating such components. As shown, the generator 14 is electrically coupled to the power converter 12 and the main or medium-voltage (MV) transformer 16. The MV transformer 16 is connected to the power grid 20 via switch gear 22. In addition, as shown, consumers are typically connected to a 690V/400V auxiliary transformer 18 and the auxiliary transformer 18 is connected to the 690V tap of the MV transformer 16. During low wind speed and/or fault conditions, the electrical system 10 is in a stand-by mode with the MV transformer 16 connected to the grid 20, which can cause undesirable power losses.
Accordingly, an improved system and method for operating a wind turbine during low wind speed conditions so as to minimize power losses would be advantageous.